In recent years, new finely working techniques have been developed as the integration degree and the performance of semiconductor integrated circuits (abbreviated to LSIs hereinafter) have been improved. Chemical mechanical polishing (referred to as CMP hereinafter) is one of the techniques, and is frequently used in LSI-producing processes, in particular, for making interlayer dielectrics flat, forming metal plugs, forming embedded wiring, and attaining others in the step of forming multilayer interconnection. This technique is disclosed in, for example, the specification of U.S. Pat. No. 4,944,836.
In order to make the performance of LSIs high, the use of copper alloy as a wiring material has recently been started instead of that of conventional aluminum alloy. However, copper alloy does not easily undergo fine working based on dry etching, which is used to form aluminum alloy wiring. Thus, the following process (the so-called damascene process) is mainly adopted: a method of depositing a thin film of a copper alloy on an interlayer dielectric in which trenches are beforehand made, so as to embed the film therein, and then removing the copper alloy thin film other than the thin film in the concave regions by CMP, thereby forming embedded wiring. The damascene process is disclosed in, for example, Japanese Patent Application Laid-Open No. 02-278822.
Before depositing a thin film of a copper alloy on an interlayer dielectric in which trenches are beforehand made, so as to embed the film therein, a barrier layer made of a barrier metal such as TaN or Ta is formed, on the interlayer dielectric, in order to prevent the copper alloy from diffusing into the interlayer dielectric, or to keep adhesive force between the copper alloy and the interlayer dielectric.
An ordinary method for making a metal such as copper alloy flat by CMP is a method of causing a polishing pad to attach onto a circular polishing table (platen), making the surface of the polishing pad wet by a polishing slurry for metal, pushing a surface of a base substance on which the metal film is formed onto the surface of the polishing pad, and rotating the polishing table in the state that a predetermined pressure (referred to as a polishing pressure hereinafter) is applied from the rear surface of the polishing pad to the metal film, thereby removing convex regions of the metal film by mechanical friction between the polishing slurry and the convex regions of the metal film.
The metal polishing slurry used in CMP is usually composed of solid abrasive particles, an oxidizer, and water. It appears that the metal film surface is first oxidized with the oxidizer to form an oxidized layer and then the oxidized layer is shaven off with the solid abrasive particles. The oxidized layer on the metal film surface in concave regions does not contact the polishing pad much, so that the shaving-off effect based on the solid abrasive particles is not produced thereon. Accordingly, with the advance of the CMP, the oxidized layer on the convex regions of the metal film is removed so that the base substance surface is made flat (see Journal of Electrochemical Society, vol. 138, No. 11, published in 1991, pp. 3460-3464).
As a method for improving the polishing rate in CMP, known is a method of incorporating an oxidized metal solubilizing agent into a metal polishing slurry (Japanese Patent Application Laid-Open No. 08-83780). It is stated that when a metal oxide shaven off with solid abrasive particles are dissolved by effect of the oxidized metal solubilizing agent, the shaving-off effect based on the abrasive particles are increased. The polishing rate based on CMP is improved by the incorporation of the oxidized metal solubilizing agent; however, when an oxidized layer on a metal film surface in concave regions is also dissolved (etched) so that the metal film surface is exposed, the metal film surface is further oxidized with the oxidizer. When this is repeated, the etching of the metal film in the concave regions advances so that the flattening effect is damaged. In order to prevent this, an etching inhibitor (protective film forming agent) such as benzotriazole is further added to the metal polishing slurry (Japanese Patent Application Laid-Open No. 08-83780). The protective film forming agent is an agent for forming a protective film on the oxidized layer on the metal film surface to prevent the oxidized layer from being etched.
FIG. 1 illustrates an example of the step of making an LSI flat. In FIG. 1(a), concave regions having a thickness B and convex regions having a thickness A1 are formed in a surface of an interlayer dielectric 1. A barrier layer 2 is formed, following the surface shape of the interlayer dielectric. Furthermore, a wiring metal layer 3 of copper, a copper alloy or the like is formed thereon. When the width of the concave regions is somewhat large, concave regions are made in the wiring metal surface, as illustrated in FIG. 1(a).
The wiring metal layer 3 other than the layer 3 in the concave regions is first removed with a polishing slurry for CMP. As illustrated in FIG. 1(b), at this time, the wiring metal layer 3 on the convex regions may be completely removed. The polishing may also be performed so as to leave the wiring metal layer 3 in a small amount.
Next, polishing slurry for barrier metal CMP is used to polish and remove the barrier layer 2 made of the barrier metal such as TaN or Ta. As illustrated in FIG. 1(c), at this time, a small amount of the interlayer dielectric 1 is polished in order to remove residues of the barrier layer 2 that may be present on the convex regions of the interlayer dielectric 1. Accordingly, the relationship between the thickness A2 of the interlayer dielectric 1 after the polishing and the thickness A1 of the interlayer dielectric before the polishing of the barrier layer is as follows: A1>A2.
Most popularly, the wiring metal layer 3 is copper or copper alloy, the interlayer dielectric 1 is SiO2, and the barrier layer 2 is a tantalum based metal such as Ta or TaN in FIG. 1. However, when the interlayer dielectric 1 is polished as described above in order to remove residues of the barrier layer 2 present on the convex regions of the interlayer dielectric 1, the interlayer dielectric 1 in a region near the wiring metal layer 3 is more largely shaven off than the interlayer dielectric 1 in other regions. Thus, there is easily caused a problem that a depression is generated. Such a depression is called a fang or seam.
FIG. 2(a) is a schematic sectional view of a wired substrate wherein lines and spaces are 100 μm and 100 μm, respectively, in width. FIG. 2(b) is an enlarged schematic view of a portion surrounded by a dot line 4 in FIG. 2(a). An ideal state after the polishing, or a case where the barrier layer 2 present on the convex regions of the interlayer dielectric 1 is completely removed so that the film can be polished into a flat form, is represented by a dot line 5 in FIG. 2(b). In general, however, such ideal polishing is difficult; usually, as illustrated in FIG. 2(b), the wiring metal layer 3 or the barrier layer 2 is somewhat excessively polished. Such somewhat excessive polishing in the wiring metal layer 3 or the barrier layer 2 does not usually become a serious problem. In accordance with the polishing property of the polishing slurry for CMP, a depression, i.e., what is called a seam 6 may be generated near the side of the wiring metal layer 3 or near an interface between the interlayer dielectric 1 and the barrier layer 2.
FIG. 3 is a schematic sectional view of a wired substrate having a fine wiring region 8, 1000 μm in total width, wherein wiring metal regions, having a width of 9 μm, are arranged to be alternated with interlayer dielectric regions, having a width of 1 μm. A depression, i.e., what is called a fang 7 may be generated near a boundary between the fine wiring region 8, wherein the density of the wiring metal regions is high, and a field region alongside the region 8.
When substrates are laminated onto each other, the upper substrate thereof follows a depression, such as a fang or seam, in the lower substrate; thus, the depression produces a bad effect on attainment of flattening the upper substrate. As the case may be, the depression causes a short circuit in wiring, so as to result in a fall in the production yield of LSIs.
An object of the invention is to provide a polishing slurry for CMP which makes it possible to polish a barrier layer made of a barrier metal such as TaN or Ta, a wiring metal layer and an interlayer dielectric continuously, and restrain, at this time, a phenomenon that the interlayer dielectric in a region near the wiring metal layer is more largely shaved than the interlayer dielectric in other regions so that a depression is generated.